It is often necessary to identify the type and concentration of various species present in an unknown sample. This is especially so when it is desired to reclaim or recycle materials for re-use. In this case, it is necessary to be able to test a product to identify the presence of recyclable species in it and their relative concentrations.
When recycling materials, it is especially important to be able to detect the presence and concentration of contaminants which would otherwise adversely affect the recycling process. Even a small level of contaminant can often greatly affect the properties of a recyclable material. However, often the physical properties (including visual) of a likely contaminant are very similar to those of a target species to be recycled. For instance, polypropylene and polyethylene have very similar properties. It may be desired to recycle polypropylene from a product. A concentration of any more than about 1% of polyethylene contaminating that product can adversely affect the moulding and other properties of the recycled polypropylene.
Thermoplastic waste is also widely perceived to be a major environmental problem. Approximately 85% of the polymers used in the EC are thermoplastics and the volume of waste materials continues to rise commensurate with materials consumption. Attention has focused on the economic treatment and disposal of waste materials from both commercial and domestic products, including packaging, white goods, scrapped automotive components and textiles.
Reclamation of plastics compositions and recycling of polymeric materials is of increasing interest to the industrial sector which includes materials waste collectors and separators, materials processors, product users, materials suppliers and processing equipment suppliers. However, with the inherent limitations in current identification and separation technology, it is inevitable that a relatively high percentage of recycled materials will contain contaminants.
Plastic bottle collection and recycling has also made considerable advances in recent years. However, to date sorting of PET (polyethylene terephthalate), PVC (polyvinyl chloride) and PE (polyethylene) bottles has been achieved mainly by hand, using unskilled labour. It is imperative that the reclaimed PET does not contain PVC, as a contaminant, in amounts greater than 50 ppm, because this results in the formation of low molecular weight degradation products which make processing of the material extremely difficult to control and can result in serious impairment of mechanical properties. In addition, the reclamation process can result in partial depolymerisation of the PET which also causes major processing problems.
In the UK, most plastics waste (90%) is disposed of in landfill sites, where it is hoped that the inert nature of plastics will have no long term harmful effects. Until recently, this approach was viewed as the optimum method of plastics waste disposal by the Waste Management Sector. However, rising costs per tonne of plastic disposed of in landfill has promoted a search for more economical solutions.
Incineration with energy recovery has been postulated as an alternative way of dealing with the waste problem. This approach, however, is viewed by many as both wasteful of a diminishing natural resource and environmentally unfriendly because of the potential impact of gas emissions. Another option is to recycle and reuse plastic waste directly.
Direct reclamation is thus increasingly viewed as the best approach to retaining the many benefits of plastics whilst minimising the impact associated with waste disposal.
However, increasing use of a wider range of thermoplastics and polymer blends with additives has made the identification and separation of waste materials and components a difficult, expensive, labour-intensive task. Moreover, at present little knowledge exists regarding the effects of recycling, and in particular, multiple recycle/processing stages on the properties of polymeric materials. The former problem reduces the cost effectiveness of producing recycled materials whilst the latter problem inhibits use of recycled materials by product manufacturers.
It is inevitable that materials currently recycled will contain contaminants, because of the inherent limitations in separation technology. For example, in reclamation of plastics materials from granulated automotive components, flotation or hydrocyclone processes should remove PVC and ABS (acrylonitrile butadiene styrene), but cannot separate PP (polypropylene) and HDPE (high density polyethylene) because they have similar densities.
Essentially, it is necessary to deal with a somewhat variable blend rather than a single homopolymer. This creates major problems for the plastics processor, because of the resultant variability in processing characteristics. In addition, PP and HDPE are incompatible, and particles of HDPE dispersed in a matrix of PP result in reduced component stiffness and creep resistance, as well as altering melt rheology.
Various methods are currently available for the identification of species in unknown samples. X-ray fluoroscopy is one such method. However, this is an expensive and inherently unsafe technique, requiring the use of protective screen systems and highly skilled operators. Moreover, its use is restricted to identifying the presence of ionic or polar species. For instance, it may be used to identify polymers such as PVC, but not electrically neutral species.
A very extensive database is needed for use with X-ray fluoroscopy if its results are to be meaningful. This database must include data for each target species which a user is likely to need to identify, for mixtures of such species and also for samples containing pigments (such as, for example, titanium oxide, used to impart a white colour) of different types and/or concentrations.
For the purposes of recycling, some target species (especially metals) can be separated from contaminants by a melting process. This technique can be used where the various species present in a sample all have different melting points; some melt before others and can thus be separately removed from the sample. However, this technique is of no use in identifying and separating polymers, which will often react with one another at or before their melting points.
Chemical reaction techniques may be used to identify some types of species from a sample. Again, however, such techniques are generally of no use for the identification and separation of polymers, the reactions of which are usually too complex to be of practical use.
Other techniques have been developed to assist in the identification and separation of plastic waste, including moulded-in codes, X-ray spectroscopy, and FT infra-red spectroscopy. However, none of these methods of approach has proved to be either completely satisfactory or acceptable to a wide proportion of the industrial waste management sector.
Moulding in bar codes seems to be the simplest, most cost effective method of approach, but this solution suffers from the possibility that in-use damage of the label would render the code unreadable. X-ray spectroscopy is a fast, reliable technique for identifying specific atomic species, such as Cl in PVC. However, the system is relatively expensive and is limited in application. It would be impossible, for example, to differentiate between PP and PE. Infra-red spectroscopy is a highly sensitive technique which can be used for detecting very small quantities of contaminant (less than 0.1 gms) in a given material. However, this approach is also expensive, and cannot easily be used on-line, because only thin film specimens can be scanned. Essentially, samples need considerable preparation prior to analysis.
Thus, there is currently much demand for the efficient separation and recycling of polymers, and yet none of the currently available techniques seems satisfactorily to meet this demand.
Ultrasonic vibrations have been used in the past to identify structural properties of certain materials. For instance, metallurgists have used ultrasound to detect the presence and geography of defects such as cracks in metals and certain polymers. These defects interrupt the passage of vibrations through the material. Such a technique has also been used in the past to study bone fractures.
Ultrasonic vibrations have not, however, been used in the past to identify a particular species in an unknown sample. There are several possible reasons for this. Every species (for instance, a chemical element, compound, alloy or mixture) will have a discrete value for the velocity at which sound waves may propagate through that species. Theoretically, therefore, it would be possible to identify a species by passing ultrasonic vibrations through a sample containing the species and measuring the time of flight for the vibrations through the sample. However, because the time of flight depends on both velocity and distance, the exact distance which the sound waves travelled through the species would also need to be measured. This distance would often be a difficult parameter to measure, particularly in the case of amorphous samples or samples of unusual shape. It would also be an impractical parameter to measure for samples in which the exact position and/or quantity of the target species within the sample were not known. For these reasons, the use of ultrasonic vibrations to identify species has until now been regarded as unacceptably impractical for use on a commercial scale.
Also seen to be a problem with the use of ultrasound is attenuation of the vibrations by the sample through which they are transmitted. Whilst on the one hand the higher the frequency of the vibrations, the higher the resolution in the transmitted signals to be analysed, on the other hand, increased frequency also increases the amount of attenuation of the vibrations. This attenuation, of the strength of ultrasonic vibrations as they pass through a sample, makes the vibrations more difficult to detect and analyse. It has historically been seen as prohibitive to the effective use of ultrasound for commercial identification purposes.
It is therefore an aim of the present invention to provide a method and apparatus for the identification of a target species in a sample, which overcome at least mitigate the above described problems with conventional methods and apparatus. They should preferably be of use for a wide range of different species types, and relatively easy, safe and inexpensive to use.